A key measure of the performance of an electromagnetic information storage system is the areal density. The areal density is the number of data bits that can be stored and retrieved in a given area. Areal density can be computed as the product of linear density (the number of magnetic flux reversals or bits per unit distance along a data track) multiplied by the track density (the number of data tracks per unit distance). As with many other measures of electronic performance, areal densities of various information storage systems have increased greatly in recent years. For example, commercially available hard disk drive systems have enjoyed a roughly tenfold increase in areal density over the last few years, from about 500 Mbit/in2 to about 5 Gbit/in2.
Various means for increasing areal density are known. For instance, with magnetic information storage systems it is known that storage density and signal resolution can be increased by reducing the separation between a transducer and associated media. For many years, devices incorporating flexible media, such as floppy disk or tape drives, have employed a head in contact with the flexible media during operation in order to reduce the head-media spacing. Recently, hard disk drives have been designed which can operate with high-speed contact between the hard disk surface and the head.
Another means for increasing signal resolution is the use of magnetoresistive (MR) or other sensors for a head. MR elements may be used along with inductive writing elements, or may be independently employed as sensors. MR sensors may offer greater sensitivity than inductive transducers but may be more prone to damage from high-speed contact with a hard disk surface, and may also suffer from corrosion, so that conventional MR sensors are protected by a hard overcoat.
Recent development of information storage systems having heads disposed within a microinch (μin) of a rapidly spinning rigid disk while employing advanced MR sensors such as spin-valve sensors have provided much of the improvement in areal density mentioned above. Further increases in linear density and track density have been limited by constraints in reducing the size of transducer features that interact with the media in recording and reading magnetic patterns. For example, inductive pole-tips and MR sensors are conventionally defined by photolithography, which limits a minimum track width for which magnetic patterns on the media can be written or read.
FIG. 1 (Prior Art) depicts a design for a thin film head 50 as would be seen from a media from which the head is to read magnetic signals. The head contains a spin-dependent tunneling magnetoresistive sensor 52 formed in a series of layers between first and second magnetically permeable shields 55 and 58 which also serve as leads for the sensor, as described in U.S. Pat. No. 5,898,548, incorporated herein by reference. The sensor and adjacent layers include a template layer 64 that helps with formation of a subsequently deposited antiferromagnetic layer 66. The antiferromagnetic layer 66 stabilizes the magnetic moment of a pinned ferromagnetic layer 68. An alumina (Al2O3) tunneling layer 70 separates the pinned ferromagnetic layer 68 from a free ferromagnetic layer 72 that has a magnetic moment that can rotate in the presence of a magnetic field from the media. A cap layer 75 of tantalum (Ta) is formed to protect the sensor from damage, and electrically conductive spacer layers 60 and 62 separate the sensor from the shields.
Formation of the above-mentioned elements begins by depositing the first shield 55, spacer layers 60 and 62, sensor layers 64, 66, 68, 70 and 72, and cap layer 75. After depositing the spacer, sensor and cap layers on the first shield 55, a photoresist 77 is lithographically patterned and the sensor is defined by ion milling material not protected by the resist. A width W0 of the sensor essentially corresponds to the width of the resist, although both may be thinned during the ion milling process. Alumina 88 is deposited to fill in around the sensor and a pair of hard bias layers 78 and 80 are formed to bias free layer 72, leaving a thick deposit of material atop the resist 77 and pointed projections 82 and 84 along the sides of the resist. The resist is chemically removed, which frees the material atop the resist 77, and the projections are broken off during chemically/mechanical polishing (CMP), after which the spacer 62 and second shield layer 58 are formed. An effective length L0 of the sensor for linear resolution is the spacing between the first shield 55 and second shield 58, which may be less than 0.1 micron.
Control of the ion milling for thinning the sensor 52 becomes difficult for widths W0 that are less than 0.5 micron, and errors in mask definition increase with mask thickness, but thicker masks are useful to over-etch the sensor to attempt to create spacer 60 out of shield 55. Therefore it has been difficult for such a prior art sensor to have a length-to-width ratio greater than 1/5. Moreover, forming spacer 62 from shield 58 requires the thin cap 75 to protect the sensor from damage during CMP, such as puncturing the cap with the broken off projections 82 and 84. Contamination such as wash chemicals or alumina from the CMP may also degrade the performance of the conductive spacer 62. While lithographic definition can be improved somewhat by using electron beam, X-ray or deep ultra violet lithography, such techniques are extremely capital intensive and require long lead times for equipment, development and facilities construction. Moreover, techniques such as X-ray and electron beam lithography are used to form individual sensors as opposed to more efficient simultaneous definition of all sensors on a wafer surface.